Background Humus serves as a crucial organic cementing agent that promotes the formation of soil aggregates. In turn, soil aggregates create a physical protective matrix for humus, thereby improving carbon retention. This reciprocal relationship establishes a dynamic equilibrium that significantly impacts soil structure and nutrient cycling. Within the riparian zone of the Three Gorges Reservoir, maintaining this balance is particularly essential due to the enhanced susceptibility to erosion and the ecological significance of carbon storage in addressing climate change. Plant communities are instrumental in enhancing both the soil aggregate stability and the capacity for soil carbon sequestration. The aim of this study is to investigate the variations in the stability and humic components of soil aggregates across different plant communities in the riparian zone of the Three Gorges Reservoir, as well as to examine the intrinsic relationships among these factors.

Methods Soil samples were obtained from three distinct plant communities (Cynodon dactylon, Xanthium sibiricum, and Hemarthria compressa) at two soil depths (0 – 10 cm and 10 – 20 cm) which corresponded to a water level elevation of 155 – 165 m within the riparian zone of the Three Gorges Reservoir. The composition of water-stable aggregates was assessed using wet sieving techniques, and the soil aggregate stability was subsequently calculated. The humic components within the aggregates were extracted using a sodium pyrophosphate-sodium hydroxide solution and quantified through the potassium dichromate oxidation method. The primary humic substances identified included humin, humic acid, and fulvic acid.

Results 1) Statistically significant differences (P < 0.05) were identified in the contents and stabilities of soil aggregates across various plant communities. Within the 0 – 20 cm soil layer, the stability of aggregates was ranked as follows: C. dactylon community > X. sibiricum community > H. compressa community, with ≥ 0.25 mm water-stable aggregate contents of 56.50%, 49.81%, and 36.38%, respectively. 2) The humic carbon content within soil aggregates in the 0-20 cm layer varied between 4.45 and 13.31 g/kg among different plant communities. In aggregates ≥ 0.25 mm, the X. sibiricum community showed the highest humic carbon content, whereas in aggregates < 0.25 mm, the C. dactylon community presented the highest humic carbon content. The PQ value of aggregates was found to be the lowest in the X. sibiricum community. 3) Correlation analyses indicated that the concentrations of humic substances and their components in micro-aggregates, as well as the presence of fulvic acid in small macro-aggregates played a pivotal role in enhancing the stability of soil aggregates within the riparian zone. A significant increase in the content of humic substances in micro-aggregates significantly improved the stability of soil aggregates in the area.

Conclusions The C. dactylon community demonstrated a greater resistance to erosion, while the soil aggregates within the X. sibiricum community displayed the lowest degree of humification. Perennial grasses were found to be more effective in resisting soil erosion and enhancing the stability of organic matter compared to annual grasses, underscoring the critical importance of judicious species selection for the ecological restoration of the reservoir riparian zone. The findings of this study may provide valuable data for informing species selection for vegetation restoration and ecological reconstruction in the riparian zone, as well as a scientific foundation for the assessment and implementation of dual carbon targets in the Three Gorges Reservoir region.